We propose to continue our studies on the spatial organization of the major cell surface phospholipids in eukaryotic cells, with particular emphasis on both their lateral and transbilayer distributions, and on the metabolism of the lipid constituents at the cell surface. Particular emphasis will be placed on the fate of exogenous lipid inserted into either the outer, or inner leaflets of the plasma membrane bilayer. This selective insertion will be accomplished by fusion of small, artificially generated unilamellar vesicles (liposomes) which are asymmetric, with the cell surface. We also propose to continue our studies on the patching and "capping" of exogenous phospholipids inserted into the plasma membranes of mouse lymphocytes. Both haptenated and fluorescently labeled phospholipids will be inserted into the lymphocyte plasma membrane by vesicle-cell fusion and exchange. Their fate will be followed using fluorescence microscopy and standard immunochemical techniques.